The five factors are: natural climate change; self-inflicted environmental damage; failure to properly respond to environmental changes; hostile neighbours; and loss of friendly neighbours. The relative impact of each factor varies for every society, and the integrated impact of the five factors can determine success or failure for that particular society. In the case of Greenland, the eventual demise of the Vikings on the island can be blamed on the confluence of these five factors. First, Greenland experienced natural climate change. Studies of ice cores show that when the Vikings arrived in Greenland, the climate was unusually warm, allowing them to grow crops and raise livestock, as they had in their ancestral Norway. But over the next 500 years, the climate in Greenland cooled so much that crops failed and livestock starved. Second, the Vikings self-inflicted environmental damage: they overgrazed the land and overharvested the trees, resulting in soil erosion. Mud samples from lake beds around the Viking settlements contained large amounts of soil, blown into the lakes during Viking times, indicating that soil erosion had become a big problem long before the region descended into a mini ice-age. Third, Greenland Vikings failed to respond and adapt to their changed environment. Their middens (waste heaps) show that they continued to eat primarily cattle, sheep, and goats instead of switching to locally available and abundant fish. Fourth, they chose not to learn about fishing from their Inuit neighbours. Rather, they considered the Inuit to be hostile neighbours and had very little contact with them, as evidenced by the lack of Inuit artifacts in their settlements. And finally, as the productivity of the Greenland Viking settlements declined, so did the trade with Europeans, resulting in a loss of friendly neighbours when they needed help the most. The Icelandic Vikings differed in significant ways. Most importantly, in the face of a changing climate and with recognition of the harmful effects of their cow-farming, wood-dependent lifestyles, the Icelandic Vikings adjusted and adapted. They switched from eating beef to eating fish, they began conserving wood, and they abandoned the highlands, where soil was especially fragile. As a result, Icelandic Vikings survived and prospered.